1. Technical Field
The present disclosure relates generally to electronic circuits for controlled energizing of light emitting diodes (“LEDs”), and more specifically for such circuits for controlled energizing of series connected red, green, blue (“RGB”) LEDs.
2. Description of the Prior Art
One of the most important functions in various portable devices such as personal digital assistants (“PDAs”), cell phones, digital still cameras, camcorders, etc. is displaying to a user the device's present condition, i.e. a display function. Without a display function, a device's user could not enter data into or retrieve data from the device, i.e. control the device's operation. Thus, a portable device's display function is essential to its usefulness.
Devices implement their display function in various different ways, e.g. through a display screen such as a liquid crystal display (“LCD”), through a numeric keypad and/or alphanumeric keyboard and their associated markings, through function keys, through an individual point display such as power-on or device-operating indicator, etc.
Due to space limitations in portable devices, these various different types of display function as well as other ancillary functions are performed largely by white LEDs (“WLEDs”) and RGB LEDs. Within portable devices, LEDs provide backlighting for panels such as LCDs, dimming of a keypad, or a flash for taking a picture, etc.
Controlling the operation of WLEDs and RGB LEDs requires using a special driver circuit assembled using discrete components or a dedicated integrated circuit (“IC”) controller. For many LEDs connected in various different ways there exists a need for a special driver circuit that provides proper power to the LEDs at minimum cost. What does proper power mean? Proper power means that the special driver circuit must provide voltage and current required so the LEDs emit light independent of the portable device's energy source, e.g. a battery having a voltage (“v”) between 1.5v and 4.2v. What does minimum cost means? Minimum cost means that the special driver circuit must energize the LEDs with maximum efficiency thereby extending battery life.
WLED Control
To permit dimming, a WLED must be supplied with a voltage between 3.0v and 4.2v and a current in the milliampere (“mA”) range. Typical WLED values for energizing the operation of WLEDs are 3.7v and 20 mA. WLEDs exhibit good matching of threshold voltage due to their physical structure. As illustrated in FIGS. 1 and 2, this particular characteristic of WLEDs is very useful for controller design.
FIG. 1 illustrates one particular configuration for a circuit that energizes the operation of parallel connected WLEDs. In FIG. 1, a battery 52 connects between circuit ground 54 and a power input terminal 56 of a conventional IC LED driver 58. The LED driver 58, which also connects to circuit ground 54, receives electrical power from the battery 52 via the power input terminal 56 for energizing its operation. For the battery polarity depicted in FIG. 1, a LED power output terminal 62 of the LED driver 58 connects in parallel to anodes 64 of several WLEDs 66. Connected in this way the LED power output terminal 62 of the LED driver 58 supplies electrical current to the WLEDs 66 for energizing their operation. To equalize or match the electrical current flowing through each of the WLEDs 66, a cathode 72 of each of the WLEDs 66 connects in series through a ballast resistor 74 to circuit ground 54. Switching the locations of the WLED 66 and the ballast resistor 74 depicted in FIG. 1 produces an electrically equivalent circuit. However, regardless of the particular circuit configuration for energizing parallel connected WLEDs 66, the ballast resistors 74 always waste power. Consequently, circuits such as that depicted in FIG. 1 having WLEDs 66 connected in parallel are an inefficient way to energize operation of WLEDs 66.
FIG. 2 depicts a number of WLEDs 66 connected in series with each other and with a ballast resistor 74. Connection of the WLEDs 66 in series is much more efficient because it limits power loss to that in a single ballast resistor 74. However, the LED power output terminal 62 of the LED driver 58 depicted in FIG. 2 must supply an output voltage that is approximately four (4) times greater than that supplied from the LED power output terminal 62 of the LED driver 58 in FIG. 1.
RGB LED Control
A LED driver 58 for RGB LEDs is slightly more complicated than that for WLEDs 66 because the three colored LEDs have different dimming threshold voltages. For example, the dimming threshold voltage for a red LED 84, such as that illustrated in FIG. 3, is approximately 1.9v, for a blue LED 94 is approximately 3.7v, and for a green LED 104 is approximately 3.7v. Resistances of three (3) ballast resistors 74 connected respectively between cathodes 86, 96 and 106 of the RGB LEDs 84, 94, 104 and circuit ground 54 must be selected accommodate the different dimming threshold voltages of the RGB LEDs 84, 94, 104. Energy dissipated in the ballast resistors 74 means that driving RGB LEDs 84, 94, 104 in parallel leads to a significant power loss.
A series connection for the RGB LEDs 84, 94, 104 illustrated in FIG. 4 reduces power loss. In the typical circuit for series connected RGB LEDs 84, 94, 104 depicted in FIG. 4, an anode 82 of the red LED 84 connects to the LED power output terminal 62 of the LED driver 58. In turn, the cathode 86 of the red LED 84 connects to an anode 92 of the blue LED 94. Similarly, the cathode 96 of the blue LED 94 connects to an anode 102 of the green LED 104. Finally, the cathode 106 of the green LED 104 connects through the ballast resistor 74 to circuit ground 54. While FIG. 4 illustrates a particular order for the RGB LEDs 84, 94, 104, those skilled in the art understand that the series connected RGB LEDs 84, 94, 104 may be arranged in any order.
An essential requirement for a LED driver 58 for RGB LEDs 84, 94, 104 intended for use in portable devices is that it be capable of supplying a specific combination of bias currents to the RGB LEDs 84, 94, 104 so they emit white light. This essential requirement for a LED driver 58 for RGB LEDs 84, 94, 104 is difficult because obtaining white light requires that a different amount of current flow through each of the RGB LEDs 84, 94, 104. The differing current requirement for producing white light from three (3) series connected RGB LEDs 84, 94, 104 prohibits using a series connection with the same current flowing through all three (3) RGB LEDs 84, 94, 104.